In their way, crossovers are just as self-indulgent as Mary Sues. The writer is having fun playing with all of her favorite toys at once. The reader, on the other hand, is saying, "What does he mean, 'Candygram'? And what's all this about horsemen?"

I'm in the "loves crossovers" camp. Love reading 'em, love writing 'em. I derive great satisfaction from weaving the different elements of the different universes together, and when they mesh well, it's pure joy. But a bad crossover makes me understand why some people refuse to read crossovers at all. I wrote one that I'm quite proud of, and it got less feedback than any other fanfic I've written. Thus, my guidelines on how to write a good crossover.

1. Introducing characters to the reader.
You're asking readers to read about characters from a show they've never watched so they can get to see the ones they do know. But giving each character a mini-biography as he makes his entrance – "This is Blair Sandburg, a health nut who was working on his dissertation in anthropology when he met…." – tends to annoy the reader. Especially the one who already knows Blair Sandburg quite well.

So how do you do it? One way is to choose one character to be the chief viewpoint character. The entire story doesn't have to be told through that character's eyes, just most of it. Thus you can depict him getting to know each of the other characters as a cover for introducing him to your readers. You can have your viewpoint character coming up against Blair's health food mania, and those who don't know Blair will learn about it at the same time, and those who do know Blair can sit back and chuckle at his in-character antics.

Another way is so easy that I felt like I was cheating when I used it on my story "The Ancient Ones Endure" . Since I was crossing over four fandoms whose twain had in most cases never met, I decided it was fair game. The fandoms were Sleepy Hollow, Highlander, Beauty and the Beast (the 80's TV show), and the Japanese comic book From Eroica With Love. I figured I could count on most readers knowing Highlander, since it's probably the most popular fandom to cross over, but I knew many fans would never have heard of From Eroica With Love, since it's rather obscure, and very few people share all of these fandoms. So at the top of the story, I put a headshot of each leading canon character and a couple of lines giving their name, what fandom they were from, and a little bit about them: "Methos, AKA Adam Pierson; Highlander TV show; the oldest Immortal, approximately 5000 years old." I felt like I was cheating, but it worked.

2. The characters don't all need to understand each other perfectly or learn everything about each other.
Some fanfics read like a group therapy session, with all the characters coming to a deep understanding of each other's motivations and issues by the end. If you are tempted to do this, please, restrain yourself. Some people will just never understand each other. And I, for one, greatly enjoy the subtle thrill of knowing something the characters don't. I once read a short Highlander/Eroica crossover, for instance, where Dorian met Methos, and it ended with Dorian not knowing about Immortals or that Methos is 5,000 years old. It was delicious. (http://ciceqi.slashcity.com/Collectors.htm)
This leads us to another vitally important point, which is:

3. Just because you love them all doesn’t mean that all of them will love each other.
In short… stories where everybody always gets along are boring. The reason I brought the Eroica characters into TAOE was that the other characters were getting along too well. They had all met and agreed to pursue their common goal together, and I needed something to keep them occupied until they caught the villain and the two characters I had decided to pair had time to fall in love. The story stalled. I put it in a drawer and took it back out a year later, by which time I had a new fandom in my life, and thought, "What if Klaus [of From Eroica With Love] showed up? He'll liven things up; he doesn’t get along with anyone." I added him, and sure enough, he got the story moving again. A professional example is that in the early 80's, DC Comics took a look at Batman and Superman, realized that these guys would never get along, and jettisoned the unconvincing friendship the two had formerly had. Think about your characters. How would they really react to each other? What personality conflicts would they have, and how difficult would it be for them to resolve them? Would they like each other, respect each other, annoy each other, or outright detest each other?

4. Look for parallel details from the different fandoms to weave together.
The best example of this I can think of is a professionally published work, Kim Newman's Bloody Red Baron. It's the sequel to Anno Dracula, in which Count Dracula succeeded in taking over England when he arrived there and created many more vampires. Both novels are full of passing references to other literary figures, such as Sherlock Holmes and Dr. Jekyll.

Bloody Red Baron opens with Dr. Jack Seward, from the original Stoker novel Dracula, disemboweling a vampiress. Soon it's made clear – he's Jack the Ripper. The common first names are not the only parallel; many who have researched the crimes of Jack the Ripper believe that his crimes display a knowledge of anatomy that suggests that he may have been a doctor or a butcher. I'm sure that not everyone who read this novel knew that bit of trivia, but those of us who did were delighted at how perfectly these details dovetailed.

5. Honor the traditions of each of the fandoms.
Depending on the requirements of the plot, one fandom may be getting more "screen time" than the others, which is fine, but your crossover should be a little more than, say, a Sentinelfic with Doyle and Bodie tossed in. (Unless their appearance is a mere cameo.) So in addition to the characters, put in the important themes of all your fandoms, even if there's only room to acknowledge them without really exploring them. In From Eroica With Love, Klaus's conflict over his own sexual orientation is a significant issue in canon. In my Eroicafics, I've delved deeply into his psyche and explored this issue in detail. In "The Ancient Ones Endure", he and his One True Love, Dorian, were supporting characters. There was no room to psychoanalyze whether Klaus is gay, why he can't accept it if he is, and how he can come to terms with it enough to finally fall into Dorian's arms. But the way he reacted to Dorian in the first scene they had together made the dynamic of the relationship clear enough. I also made the commitment both Ichabod Crane and Detective Diana Bennett (from BatB) have to scientific crime detection, and to criminal justice, important to the story. Neither character would be themselves without that.

Lesser details should also be included. Since BatB's Vincent often quotes literature and poetry in canon, I had him do so in my story. Another example: if one of your crossover fandoms is The X-Files, have a clock say 11:21 at some point. (It's Mrs. Chris Carter's birthday, that's why he kept sneaking it in.)

6. Don't have one character solve all the problems or make all the mistakes.
You're allowed to have favorites. I'm sure you do have a favorite. But if one character is consistently right about everything, smarter than everybody else, and rescues everybody else constantly, he's going to be as annoying as a Mary Sue. Let somebody else win sometimes. The same goes for having one character constantly screw up. You may need someone to make mistakes to keep the story going, but it shouldn't always be the same person. You may hate Canon Character X and regard him as the scum of the universe, but if all the problems in the story are his fault, the reader's inevitable question is, "Why didn't they just get rid of him on the first page?" Think Joxer. Think Jar-Jar. Not to mention that you'll alienate the readers who like Character X.

7. Plot each thread as if it were its own story.
Unless your story is short and/or simple, with all these different characters it's going to involve a few storylines woven together. Some will doubtless be more important than others, or take up more space, but the only way not to let one character or fandom be slighted is to work out the entire story for each of them. What would each character be likely to be doing each step along the way of the plot?

Vickie Wyman wrote two Eroica/Lupin crossovers. (Both are available from Anime House through (marg@golden.net.) Lupin is her main fandom, and she writes Eroica's Dorian very well, but her Klaus is kind of shaky. In the first crossover she wrote, "Thoughts Contingent on a Blithe Spirit", Klaus agrees to allow Lupin to resolve the situation in his own fashion. Never mind that Lupin is a professional thief and Klaus a NATO operative – i.e., a law enforcement professional – and a textbook case of a control freak! But she wanted her beloved Arsene Lupin to save the day, not Klaus. In her second crossover, "Thoughts Contingent on the Wrong Box", she seemed to realize that Klaus wouldn't sit back and let other people run things as long as he was conscious, so she gave him a severe concussion and kept him in a coma for most of the story. These are both good stories, but they are not the best examples of what crossovers should be.

8. Have canon betas.
If you're not a canon expert on all of the fandoms you're writing about, find someone who is. I only have a general acquaintance with Highlander, so when I included HL's Amanda in my Eroicafic "The Picture of Dorian Red" , I sought out Highlanderfen to consult with. I asked them several questions while I was working on the story. I still used my usual betas for general readability, but I also used these new HL betas to tell me whether my depiction of Amanda's personality and mundane details were accurate. Good thing, too, because I misunderstood one of the answers I had gotten and made one major mistake about her, and was able to correct it before unleashing the story to the Net.

Think of it: you've worked hard on your Professionals/Sentinel crossover. A Sentinel fan decides to give it a try. She's being drawn in by your masterful characterization, plotting, and style… she's even thinking about reading your Pros stories and maybe joining that fandom… until "What the heck is Blair doing eating a Big Mac?!?" and you've lost her.

With all this… why crossovers? One reason, of course, is that they can be a lot of fun. Another is that adding another fandom can make a story work. Though not necessarily. As I mentioned earlier, my SH/HL/BatB crossover was stalled until it occurred to me to introduce the Eroica characters into it. Once I did that, it was finished in a couple of weeks.

Right now I'm at work on a crossover between a mere two fandoms, and I'm sweating a great deal over not slighting any important characters as it is. (Three of the four canon characters are law enforcement professionals, which makes it difficult to keep the one who's not in the loop.) So a multiple crossover is exceedingly challenging. Proceed with caution, but it's worth thinking about.